The present invention relates generally to input protection circuits and more specifically to an improved electrostatic discharge protection network for insulated gate field effect transistor circuits.
The occurrence of spikes, noise or other high voltage undesirable signals on the input of transistor circuits are well known. The prior art has attempted to provide protection for these circuits by using a resistor-diode input protection circuitry. If the transistors to be protected are formed in an N-type substrate for example, a P-type diffused resistor is formed in the N-type substrate and connected between the input to the transistors and the input to the circuit. The P-type resistor forms a PN junction with the substrate. By the use of a single diffused resistor, the input resistance as well as the breakdown voltage of the diode is determined by the same diffusion.
Attempts have been made to distribute the resistance and breakdown voltage between the input and the output end of the resistor by placing a higher impurity concentration N region at the output side of the resistor. A typical example is in U.S. Pat. No. 3,748,547. The placement of the increased impurity N region is critical since its distance from the input is used to define the breakdown voltage as a function of the spread of the depletion region. Other attempts at lowering the breakdown voltage of the resistor-diode input circuit using P+ and N+ contacts to a P- resistor is described in U.S. Pat. No. 3,673,428. By using a P- resistor to effectuate both of the breakdown adjustments, the resistor has poor temperature coefficient and therefore is undesirable. Similarly, there is no SCR protection from the ground diode 30 which causes latch-up of the circuit.
A resistor-diode input protection circuit which addresses the problem of latch-up is described in U.S. Pat. No. 4,143,391. An SCR Q5 and Q6 is intentionally formed at the output side of the input resistor. By designing the circuit without a resistor at the base of Q5, the SCR will not latch up. To prevent the formation of an SCR between the input resistor and the well in which one of the complementary transistors is formed, the base region of transistors Q7 and Q8 are formed to a large enough length to produce a low current amplification factor Beta. Thus, the elements in this circuit must be at specific locations and distances and thereby limit the circuit design.
Although addressing single portions of the input protection circuit, the prior art has failed to address substantially all the issues involved in designing a resistor-diode input protection circuit.
Thus, it is an object of the present invention to provide an improved resistor-diode input protection circuit which addresses substantially more of the desired characteristics than the prior art.
Another object of the present invention is to provide a resistor-diode input circuit having a high breakdown voltage-high resistance input portion and a low breakdown voltage-low series on resistance at its output portion.
Still another object of the present invention is to provide an improved resistor-diode input circuit which prevents SCR formation and latch-up while allowing the designer freedom in the circuit layout.
A further object of the present invention is to provide a resistor-diode input protection circuit wherein the series resistance and the breakdown voltage may be separately optimized.
An even further object of the present invention is to provide a resistor-diode input protection circuit having a low input capacitance.
These and other objects of the present invention are attained by forming the input resistor of two diffused regions with the higher impurity concentration region being formed in a deeper and wider low impurity concentration diffused region. This allows the high impurity concentration region to define the series resistance of the input resistor and the low impurity region to substantially define the breakdown characteristic of the input diode. A high impurity concentration of the same conductivity type as the substrate extends into the low impurity concentration of the resistor at the output or transistor side of the resistor diode input circuit, to provide a low breakdown voltage-low series resistor-diode combination at the transistor side. This increased impurity concentration region of the same impurity type as the substrate extends adjacent to and substantially along the remainder of the low impurity concentration portion of the resistor region.
A first guard ring having an impurity concentration opposite that of the substrate completely surrounds the resistor-diode input protection circuit and isolates it from the transistor device to which it is a protection device. The first guard ring includes a deep low impurity concentration region in which is formed a higher impurity concentration region of the same conductivity type. A second guard ring of a higher impurity concentration than and the same conductivity type as the substrate is formed laterally separating the input protection circuitry and the transistor to which it is protecting. The second guard ring is interior the first guard ring. First and second surface conductors make contact with substantial portions of the first and second guard rings respectively. The resistor-diode input circuitry also includes a vertical bipolar transistor of which the substrate is a collector and including a collector contact which is part of the second guard ring and completely encompasses the base of the vertical bipolar transistor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.